1. Field of the Invention
The present invention refers to a current generator with thermal protection. Particularly it refers to a generator able to sustain voltages applied to it superior to 400 V. Besides it refers to a power supply of the pulse width modulation type comprising a current generator with thermal protection used as start-up of the power supply.
2. Description of Related Art
In the current generators only one feeding voltage is usually available and no additional feeding voltages are used as they would increase the complexity of the circuit and its cost. Besides, in order that a current generator can be considered as such, the necessary current for its working must be negligible compared to the one supplied. In such a case it becomes problematic to realize a circuit inside that, by measuring the temperature of the current generator, switches it off when the latter exceeds a preset value, as this circuit would make increase the bias current used for the circuits inside the current generator. The difficulty increases when the current supplied by the current generator decreases with the consequence of having to decrease the bias current of the circuits set inside the current generator so that the current generator is as ideal as possible. For instance with a current supplied Iout=1 mA and a bias current Ibias included between 20 and 50 xcexcA it is almost impossible to realize a circuit able to measure the temperature and to switch off the generator if necessary.
In view of the described prior art an object of the present invention is that of providing a constant current generator with the varying of the supply voltage having only one supply voltage and a thermal protection circuit which has not got the drawbacks of the known art, and besides a pulse width modulation power supply having a current generator of which above.
In accordance with the present invention, these and other objects are reached by means of a current generator with thermal protection having an input terminal and an output terminal comprising: a voltage generator; a first controlled switch having a control terminal applied to said voltage generator, a first terminal connected to said input terminal, and a second terminal connected to a resistance; characterized by comprising: a second controlled switch having a control terminal coupled to said voltage generator, a first terminal connected to said resistance, and a second terminal connected to said output terminal; a temperature sensor of said current generator able to measure the temperature of said generator; a control circuit of said second controlled switch able to open said second controlled switch in the case in which the temperature of said current generator overcomes a preset temperature.
In accordance with the present invention, such objects are reached also by means of a pulse width modulation power supply comprising a current generator in accordance to claim 1 used as start-up circuit.